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GUIDING (JTR March 2001) CONTENTS
  1. INTRODUCTION
  2. IR GUIDING USING SPEX GUIDER/SLIT-VIEWER
  3. VISIBLE GUIDING USING THE OFF-AXIS GUIDER
  4. VISIBLE GUIDING USING THE TIP-TILT SYSTEM

  1. INTRODUCTION
    
    To maximise throughput of the spectrograph it is important
    to keep the science object in the slit. Since SpeX is
    optimised to use slit widths about the same size as the
    seeing it is necessary to use active guiding rather than
    rely on telescope tracking alone.
    
    OBSERVERS SHOULD COME PREPARED WITH SUITABLE GUIDE STARS.
    
        
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  2. IR GUIDING USING SPEX GUIDER/SLIT-VIEWER
    
    
       Guiding can be done on spill-over flux from 
       the object in the slit or on an object in the 
       60x60 arcsec field of the imager. The magnitude
       limit for auto-guiding on a star in the slit
       is about J=15. The magnitude limit for manual
       guiding (using the paddle) on a star in the slit
       is about J=18. The magnitude limit for auto-guiding
       on a star in the field is about J=18. These limits
       depend on seeing and slit width.
    
       Auto-guiding also works very well on extended objects
       such as galaxy nuclei and small disks (eg. 4 arcsec diameter
       Uranus) so long as the objects are bright enough and
       guiding on the peak or centroid is acceptable. Positioning
       a slit at a particular location on Jupiter or Saturn, for
       example, must be done using non-sidereal telescope tracking
       with manual updates provided by observing the slit location
       in the SpeX slit-viewer. 
    
       For details read the document 'How to set up the IR guider'.
    
       
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  3. VISIBLE GUIDING USING THE OFF-AXIS GUIDE
    
    
       Visible guiding can be done using the off-axis guider.
       This guider has an annular field-of-view with an inner
       radius of 100 arcsec and an outer radius of 200 arcsec.
    
       Magnitude limit     Itime
          (no moon)        (sec)
           V=10.0           0.7
           V=12.7           3.5
           V=14.0           6.5
           V=15.0           8.0
    
       Place the science object in the slit and ask the telescope
       operator to start guiding on a suitable off-axis star. 
       Make sure that the science object does not move out of the
       slit when guiding is started (it should not). Set up the
       telescope nod position as described in 
       'How to set up the the IR guider'.
    
        
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  4. VISIBLE GUIDING USING THE TIP-TILT SYSTEM
    
    
       Visible guiding can be done using the tip-tilt guider.
       This guider has an 80 x 80 arcsec FOV. Due to the
       fragility of the tip-tilt wavefront sensor package
       tip-tilt guiding has seen little use and we urge
       observers not to assume it will be working.
    
       In the SpeX XUI move a dichroic into the beam. There is
       a choice of two:
       1) 0.80 micron cut-on which sends wavelengths < 0.80 micron
          to the wavefront sensor (less light to guide on but
          spectral coverage > 0.8 micron)
       2) 0.95 micron cut-on which sends wavelengths < 0.95 micron
          to the wavefront sensor (more light to guide on but 
          less spectral coverage > 0.95 micron)
    
       Place the science object in the slit and ask the telescope
       operator to start guiding on a suitable off-axis star.
       Make sure that the science object does not move out of the
       slit when guiding is started (it may need slight repositioning
       using tip-tilt guide cursor buttons - ask TO). Set up the
       telescope nod position as described in 'How to set up the
       the IR guider'.
    
    
    
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